Accelerometer sense element

Abstract: A sense element is provided which has a boron doped silicon body (12) positioned above and parallel to a substrate (10) with a plurality of openings (50,51) extending through the semiconductive body, the plurality of openings (50,51) producing a damping such that the natural frequency and the measurement bandwidth of the sense element are modified. The body (12) is rotatable about a flexure axis (28) located above and parallel to the substrate (10).

Description: This invention relates sense elements for use in accelerometers. Transducers which translate nonelectrical quantities into electrical signals have long been known. These devices are designed to provide a usable output in response to a particular physical quantity, property or condition which is desired to be measured. The term 'sense element' has been used to define both the overall transducer as well as the transducer element which performs the first step in a multistep translation process. For the purposes of the instant invention, the latter definition will govern. Transducers with sense elements used to measure acceleration are known as accelerometers. Accelerometer applications in the automotive industry include incorporation into crash sensors for air bag deployment and ride motion sensors for active suspension components. Accelerometers also find use in the military and aerospace industries as rate of climb indicators and as components in smart weapons and missile arming devices. Prior art sense elements have had difficulty in obtaining and maintaining the strength and response characteristics required by the rigorous working environment of such accelerometers. These sense elements relied heavily on cantilevered beams, seismic masses, and cylinder/ball arrangements affected by fluid pressure and travelling friction. These devices suffered from extreme fragility and breakage problems. In addition, it has often been difficult to make these prior art sense elements efficiently and economically, due to the rigours of large scale manufacturing processes. Finally, these devices often exhibit excess and undesired sensitivity in orthogonal axes. The present invention is concerned with a capacitive sense element for an accelerometer, comprising a substrate having a substantially planar upper surface with at least two fixed conductive plates thereon and a substantially planar deflectable body positioned above and parallel to the substrate, the body comprising: a) a pedestal attached to the substrate and surrounded by an internal opening in the body; b) a first upper plate and a second upper plate corresponding to the fixed conductive plates to form a first and a second capacitor; and c) a pair of oppositely directed torsion arms connecting the body to the pedestal and forming a flexure axis around which the first and second upper plates deflect, the total moment of the second upper plate about the flexure axis being greater than the total moment of the first upper plate about the flexure axis such that the body deflects in response to an acceleration normal to the sense element to produce a difference between the capacitances of the first and second capacitors. US-A 4 736 629 issued to Cole (the '629 patent) describes an accelerometer having a sense element of this kind in which the upper planar body is a metallic plate with an internal opening surrounding a pedestal mounted to a substrate of a semiconductive material such as silicon. The pedestal is connected to the metallic plate by a pair of torsion members extending in opposite directions from the pedestal to the metallic plate. Fixed plates positioned on the semiconductor substrate and covered with an insulating layer correspond to portions of the metallic plate to form first and second capacitors. In response to acceleration normal to the substrate the metallic plate rotates around the flexure axis defined by the torsion members to vary the capacitance of the first and second capacitors. The '629 patent expressly requires that the rotatable upper plate be made of metal and the substrate be a semiconductive material such as silicon. Although Cole teaches that the electrically conductive substrate will be covered with an insulating layer, the conductive nature of the substrate renders the disclosed sense element vulnerable to parasitic capacitance loss. As a result, Cole's sense element is potentially less accurate and less sensitive. Cole also teaches that forming inertial elements from low density materials like silicon or silicon dioxide results in sense elements with less sensitivity to acceleration as compared with corresponding sense elements with inertial elements made of metal. The disclosure teaches that such semiconductive constructions are non-feasible. Although admitting that the coefficient of thermal expansion of metal is significantly greater than that of silicon, Cole postulates that the structure of the '629 accelerometer sense element eliminates any bias due to differential thermal expansion or contraction. However, Cole fails to account for the effect of undesirable compressive stress induced in the torsion arms as a result of differential thermal expansion between the supporting pedestal and the underlying substrate. In addition to undesirable stresses in the torsion arms, Cole's metallic upper plate is highly vulnerable to hysteresis and creep as well as plastic deformation from mechanical stresses. It will be appreciated that hysteresis defines lags or delays between the onset of acceleration and its resulting effect, i.e. the change in the relative capacitances, and is highly undesirable in a sense element intended for use in applications requiring a high degree of accuracy and reliability, i.e. crash sensors for air bag deployment. Even more importantly, Cole teaches that the sense element should be critically damped. However, this results in a sense element with a mechanical sensitivity which decreases the device's shock survivability. It will be appreciated that these characteristics make the use of a critically damped sense element highly undesirable in a crash sensor since such devices may be responsive to undesired signals. According to the present invention, in a capacitive sense element for an accelerometer of the kind referred to above the substrate consists of a dielectric material and the body consists of boron-doped silicon having a single-crystal structure and has a plurality of openings extending through it, the openings producing a damping effect modifying the natural frequency and the frequency response of the sense element such that the sense element exhibits a damping ratio of from 2 to 20 and a measurement bandwidth of 0 to 5000 Hz. It has previously been proposed in DE-A-42 24 383 and GB-A-2 158 945 to form openings through a deflectable silicon electrode of a capacitive sense element for an accelerometer in order to reduce damping, but in these prior disclosures a single electrode was supported between upper and lower fixed metal plates with a very narrow gap both above and below the electrode. A sense element embodying the invention reduces the problems of the prior art, particularly with respect to the strength, survivability and response characteristics required of sense elements for accelerometers in general and particularly those used in automotive crash sensors for air bag deployment. In addition to increased shock survivability a sense element embodying the invention has a structure and composition which substantially eliminates undesirable stresses resulting from differential thermal expansions or contractions and is free from hysteresis. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the top view of the sense element portion of an accelerometer utilizing two sense elements of the instant invention; Figure 2 provides a perspective view of the sense element of the instant invention; Figure 3 illustrates a cross-sectional view of the sense element of Figure 2 taken along the line 3-3; Figures 3A-3C illustrate the sense element of Figure 3 at three stages in the process of manufacturing; Figure 4 provides a side-cut-away view of the sense element of Figure 2 taken along the line 4-4; Figure 4A illustrates the sense element of Figure 4 at one stage of the manufacturing process; Figure 5 provides a side-cut-away view of the sense element of Figure 2 taken along the line 5-5; Figures 5A and B illustrate the sense element of Figure 5 at two stages in the process of manufacturing; Figure 6 illustrates a sense element structurally similar to that of the invention but without the through openings in the upper plates; and Figure 7 shows an expanded view of the circled portion of Figure 5B. This invention provides sense elements which are intended for in use in accelerometers generally and in crash sensor applications in particular. Advantages such as freedom from plastic deformation and stresses induced by differential thermal expansions and contractions, increased strength and improved manufacturing efficiency and economy are achieved with the use of a sense element comprised of a dielectric substrate and a boron doped silicon deflectable upper body. Prior art problems such as shock survivability and fragility have been solved with a unique structure which provides a sense element which is responsive to accelerations in the range of 0.5 to 1000 g, has a measurement bandwidth from 0 to 5000 Hertz, and a damping ratio from 2 to 20. Figures 2, 3, 4, 5 and 7 illustrate a preferred embodiment of the claimed sense element. Figure 6 illustrates a sense element similar to that of Figure 2 but without through openings in the upper plates. Figure 1 illustrates the sense element portion of an accelerometer comprised of two of the sense elements of the instant invention. Turning to Figure 2, it will be seen that sense element 8 is a capacitive sense element comprised of substantially planar substrate 10 and deflectable upper body 12 which is positioned above and parallel to the substrate 10. Substrate 10 is a solid dielectric material such as glass, ceramic, nitride or plastic. Most preferably, substrate 10 will be glass and will have a coefficient of thermal expansion very near that of silicon. A suitable glass for use as substrate 10 is Corning Code #7740 which is available from Dow Corning, located in Corning, New York, U.S.A. Substrate 10 will preferably have a thickness from 0.2 to 1.5 millimeters and most preferably will be from 0.5 to 0.8 millimeters thick. Fixed conductive plates 16 and 18 are mounted on the substantially planar upper surface 14 of substrate 10 and may be seen in phantom in Figure 2 or in the side views of Figures 4 and 5. Fixed conductive plates 16 and 18 are comprised of metal and are preferably equal to one another in size and shape. Most preferably, they are positioned symmetrically with respect to flexure axis 28 and are smaller than upper plates 38 and 40. As will be discussed in greater detail below, upper plates 38 and 40 are those portions of deflectable upper body 12 which correspond to fixed plates 16 and 18 to form first and second capacitors. While a variety of metals are suitable for the formation of fixed plates 16 and 18, a combination of metals will be preferably used. A suitable combination is chromium underlying platinum wherein chromium will have a thickness from 50 to 1000 Å and platinum will have a thickness from 50 to 3000 Å . Preferably, the chromium layer will be 150-300 Å thick and the platinum layer 500-1500 Å thick. Most preferably the chromium layer will 200 Å thick and the platinum layer 1000 A thick. Those skilled in the art will appreciate that the total metal film thicknesses will range from between 100 Å to approximately 4000 Å. Also lying on the surface 14 are conductors 20 and 22, which connect fixed plates 16 and 18 to the rest of the transducer or accelerometer. While these electrical connections are also formed of metal, it is most preferred that they be of aluminum. Turning briefly to Figure 1, it can be seen that conductors 21 and 23 are analogous to conductors 20 and 22. Figure 1 also illustrates that conductors 21 and 23 connect to bond pads 24 and 26 from which conductors 21a and 23a ultimately connect the sense element 8 to the integrated circuitry which comprises the remainder of the accelerometer or transducer. Returning to Figure 2, deflectable body 12 is substantially planar and is positioned above upper surface 14 of substrate 10 so as to be parallel to upper surface 14 in the absence of both gravitational and acceleration forces normal to surface 14. Deflectable upper body 12 is comprised of pedestal 30 which is surrounded by internal opening 32 and connected to main body 34 by torsion arms 36. Main body 34 is substantially planar as discussed above and is comprised of upper plates 38 and 40 which are respectively positioned above fixed conductive plates 16 and 18 to form a first capacitor and a second capacitor. Deflectable upper body 12 will have a length from 0.1 to 10 millimeters and a width of approximately 0.05 to 5.0 millimeters. Most preferably, deflectable body 12 will have a length of approximately 1 millimeter and a width of approximately .5 millimeters . Semiconductive deflectable upper body 12 will preferably have a shape such that upper plate 38 will have a total moment (i.e. mass times moment arm) about flexure axis 28 which is less than the total moment of upper plate 40. Thus, it will be appreciated that in response to acceleration normal to upper surface 14, deflectable body 12 will rotate about flexure axis 28, the degree of rotation or deflection being approximately proportional to the magnitude of the acceleration, and the degree of rotation or deflection corresponding to the direction of the acceleration. Torsion arms 36 are positioned within internal opening 32 to connect pedestal 30 to main body 34 of semiconductive body 12. Torsion arms 36 form the flexure axis 28. In response to acceleration normal to upper surface 14, upper plates 38 and 40 will deflect or rotate about flexure axis 28 to produce a difference between the capacitances of the first and second capacitors respectively formed by fixed conductive plate 16 and upper plate 38 and fixed conductive plate 18 and upper plate 40. Turning briefly to Figure 3, it can be seen that pedestal 30 is attached to upper surface 14 of substrate 10. As will be discussed below, most preferably, pedestal 30 will be anodically bonded to substrate 10. Set within recess 42 of pedestal 30 is metal bond pad 44. Bond pad 44 most preferably will be of aluminum. Turning to Figure 1, it can be seen that bond wire 46 connects bond pad 44 to bond pad 48. Conductor 48a leads to the integrated circuitry discussed above. Note that the various bond wires, bond pads etc. have been omitted from Figure 2 to better illustrate other aspects of the claimed invention. As illustrated in Figure 6, deflectable upper body 12 is comprised of boron doped silicon. The use of boron doped silicon substantially eliminates the prior art problems of hysteresis and creep to increase the overall accuracy and reliability of sense element 8'. The density of the boron doped silicon used herein provides a sense element wherein upper plates 38 and 40 have sufficient mass and moment arm such that the sensitivity of sense element 8 can be controlled and a desirable level achieved. The silicon of deflectable body 12 will preferably have a boron concentration of 1 x 10 19 cm -3 or greater. Turning to Figure 4, the boron concentration should be symmetric with respect to a plane substantially parallel to the surface of upper plates 38 and 40 and lying at a point of approximately 1/3 of the total desired thickness of deflectable body 12. While there may be some change in boron concentration over the thickness of deflectable body 12, the boron concentration should not be less than 1 x 10 19 cm -3 . The higher the differential mass of upper plates 38 and 40 the greater the sensitivity of sense element 8. The use of boron doped silicon to form deflectable body 12 not only provides the performance advantages of the metallic upper plate disclosed in the '629 patent, but also eliminates problems such as hysteresis and creep as well as non-desirable plastic deformation resulting from high stress conditions and mechanical forces. Hysteresis is eliminated because a single-crystal silicon wafer with no grain boundaries is utilized for the formation of the boron doped silicon deflectable body 12. The unique property of silicon to exhibit no plastic deformation until catastrophic failure is responsible for the elimination of creep. The use of the boron doped silicon and the dielectric substrate substantially reduces stresses caused by the use of materials with significantly different coefficients of thermal expansion. The single crystal structure of the boron doped silicon has the further advantage of being non-magnetic and more chemically stable and inert as compared to metals and polycrystalline materials. It is also more mechanically stable than such materials. The mechanical construction of the sense element as described up to this point is illustrated by Figure 6 as sense element 8'. However, another aspect of the claimed invention will now be described which is not illustrated in Figure 6. Inasmuch as the sensitivity of sense element 8 corresponds to the respective masses of upper plates 38 and 40, it is advantageous for said upper plates to be as large as possible. However, turning to Figures 4 and 5, it will be seen that the gap 48 between deflectable body 12 and upper surface 14 is very small relative to the dimensions of upper plates 38 and 40. Indeed, gap 48 will be approximately 1 to 10 microns. As a result, substantial damping effects result from the tendency of the air to resist displacement when 'squeezed' between surface 14 and upper plates 38 and 40. While the prior art teaches that "compromises among sensitivity, resonant frequency, and damping of undesirable resonances" are required of all accelerometer designs, it also teaches that increased sensitivity generally results in "lowered resonance frequency . . . [where] . . . the lowest resonance limits the available measurement bandwidth." See "A Monolithic Silicon Accelerometer With Air Damping And Over Range Protection", P.W. Barth et al.; Proceeding of IEEE Solid-State Sensor And Actuator Workshop, 1988, 88th 0215-4, pp. 35-38. Sense element 8', however, as illustrated in Figure 6, has a desirable sensitivity but a damping ratio which produces a measurement bandwidth which is too narrow and a natural frequency which is too low. Such "response" characteristics are undesirable because the sensor response is too slow and is not responsive to the desired frequency components of an acceleration signal. This is particularly true with respect to sense elements for accelerometers intended for use in automotive crash sensors for air bag deployment. Thus, although a desirable sensitivity has been achieved, the measurement bandwidth of the embodiment illustrated in Figure 6 is too narrow and the natural frequency too low for some applications. Turning to Figure 2, to achieve a sense element having desirable sensitivity as well as particular response characteristics, a plurality of openings 50 is extended through the deflectable body 12. Preferably, the plurality of openings 50 will extend through the thickness of those portions of deflectable body 12 which rotate in response to an acceleration normal to upper surface 14. The openings 50 will thus most preferably extend through upper plates 38 and 40. Note that although Figure 2 illustrates the plurality of openings 50 as individual circular holes 51, other shapes and configurations are possible. In Figures 4 and 5 the individual holes 51 can be seen. Note that the holes are most preferably staggered as illustrated by the phantom holes 51' in Figure 4. The individual holes 51 may also be seen in Figure 5. Note, that a greater number of holes having relatively small diameters from 0.1 to 50µm is much preferred compared to fewer holes of greater diameter. The greater the diameter of the individual holes 51, the greater the chance that errors in the total capacitance will result. Those skilled in the art will appreciate that manufacturing control of large holes is more difficult than for small. Thus, most preferably, the diameter of individual holes 51 will be minimized and be from about 0.1 to 50µm. The use of a sense element structure incorporating the plurality of openings 50 achieves multiple effects not possible in previous prior art sense element structures. Unlike the internal opening 32 which surrounds the pedestal 30 and is necessary to allow the deflectable body 12 to rotate about flexure axis 28, the plurality of openings 50 change the mass and the total moment of upper plates 38 and 40. As a result, the natural frequency is increased and the measurement bandwidth widened. However, in addition to changing the mass, the plurality of openings 50 also affects the air flow of the air surrounding sense element 8. The 'viscous' air previously trapped between the upper plates 38 and 40 and surface 14 can now flow upward through the individual openings 50. As a result, the overall damping ratio of sense element 8 is decreased and the speed of the sensor response increased. A final advantage of using the plurality of openings 50 is that the number and size of openings 50 can be easily controlled during manufacture so as to achieve a particular set of desired response characteristics. As a result of this unique balance between sensitivity and response characteristics, the sense element 8 will not be responsive to its resonance frequency of about 1-5 KHz in the working atmosphere discussed below. Another significant advantage of sense element 8, is that the high damping ratio produces a sense element with increased shock survivability. When sense element 8 is dropped no deformation or damage should occur to deflectable upper body 12. This effect is supplemented by the substantial resistance of the boron-doped silicon to suffer plastic deformation caused by mechanical stress. It is anticipated that the sense element disclosed herein will survive not only the rigors of use, but also the traditional hazards associated with the manufacturing of the sense element and the subsequent processing required for its final incorporation into an accelerometer used in a crash sensor, automobile, airplane, etc. It is intended that the disclosed structure and composition result in a sense element 8 which suffers no loss of sensitivity from mechanical shocks and will maintain its original state without deformation or damage which would reduce the sensitivity and/or accuracy of the device. Ideally, in the event of a shock of sufficient magnitude the sense element 8 will break so as to be non-functional rather than function and thus provide inaccurate signals while deformed or bent. As previously mentioned, a significant advantage of the claimed sense element over prior art sense elements is the efficient and cost effective manufacturing process used to make it. This method of making itself contributes to a sense element 8 with increased strength and higher reliability than prior art sense elements. The method of manufacturing the claim sense element will now be disclosed with the use of Figures 3, 3A-C, 4, 4A, 5, 5A and 5B. Note that while these figures illustrate the process of manufacturing from three different views of sense element 8, they do not attempt to illustrate every step of the manufacturing process. Also, the dimensions of certain aspects of the drawings have been exaggerated for the purposes of clarity. Thus, the aspect ratios of the device are not intended to be illustrated therein. Turning to Figures 3A and 5A, the method of manufacturing sense element 8 begins with the selection of a single crystal silicon wafer 52 preferably having a 100 crystal orientation and polished on a single side. Although Figures 3A and 5A only show wafer 52 after the first etch step, those skilled in the art will appreciate that the original state of such wafers is known. Wafer 52 is initially from 0.3 mm to 0.7mm thick, with a diameter from 75 mm to 200 mm or greater. Most preferably the thickness of the wafer 52 will be about 0.4-0.5 mm. The thickness should increase with increasing diameter. After conventional cleaning, the wafer 52 is thermally oxidized to form a silicon dioxide (SiO 2 ) layer (only portion 54) with a thickness of approximately 1,000 to 7,000 Å and most preferably 3000 to 4,000 Å. The SiO 2 layer will serve as a mask for a subsequent anisotropic silicon etch. The wafer 52 is then patterned using conventional photolithographic techniques so as to leave SiO 2 portion 54 where desired. After conventional cleaning a hydrofluoric acid dip is used to remove any 'native' SiO 2 which may have grown on SiO 2 portion 54. Silicon wafer 52 is then etched in anisotropic etchant such as a KOH/NaOH solution, to remove a thin section of bulk silicon not covered by SiO 2 portion 54. Approximately 0.5 to 10µm. of silicon, preferably 5.0µm, will be removed. A preferred anisotropic etchant is a KOH/NaOH solution containing three kilograms KOH and .316 kilograms NaOH in 6 liters of deionized water. Wafer 52 remains in contact with the isotropic etchant solution for a time of sufficient duration to provide the desired recess. Preferably, the wafer 52 will remain in contact with the solution for between 5 to 30 minutes at 70 to 90°C and preferably for 10 to 20 minutes at 60°C. Most preferably the contact will be for about 15 to 17 minutes at 60°C. The resulting structure 62 is pictured in Figure 3A. Note that the dotted line in Figure 5A shows the original thickness of silicon wafer 52 prior to the isotropic etch step. SiO 2 portion 54 is then stripped using concentrated hydrofluoric acid. Remaining silicon wafer 52 and structure 62, see Figures 3A and B, are then cleaned and a diffusion mask of silicon nitride deposited. This diffusion mask is deposited using conventional low pressure chemical vapor deposition (LPCVD) techniques and should have a thickness from 300 Å to 3,000 Å and preferably a thickness of 500-1500 Å . Using conventional photolithographic techniques, a mask is patterned on the silicon nitride layer and followed by a plasma etch to remove silicon nitride from those areas where boron diffusion is desired. Silicon nitride mask 56 may be seen in Figures 3B, 4A and 5B. Note that careful control of mask opening 68 in Figure 5B is necessary to achieve a satisfactory balance between maximization of the mass of upper plates 38 and 40 and the torsional rigidity/strength of torsion arms 36. The torsional rigidity of torsion arms 36 is approximately inversely proportional to the cube of the thickness of torsion arms 36. However, the shape of the torsion arms 36 also influences this characteristic. The cross sectional shape of torsion arms 36 is a function of the diffusion mask openings and the diffusion process itself. In general, the width of mask opening 68 should always be less than the boron diffusion depth 70 of the "bulk" areas 72 which will become main body 34 and upper plates 38 and 40. In the bulk areas 72 the boron diffusion depth is independent of the mask opening 74. However, with respect to mask opening 68, the depth of the boron diffusion is dependent on the size of mask opening 68. Thus control of mask openings 68 and 74 allows for greater control in the formation of torsion arms 36 and hence greater control of sense element 8's sensitivity to acceleration. The control of silicon nitride mask opening 74 does influence the depths of the lateral and vertical boron diffusion in the bulk areas 72. Lateral diffusion produces rounded edges 76, as illustrated in Figures 5B, 5 and 7. Although the aspect ratios in the drawings have been exaggerated for the purposes of clarity, the lateral boron diffusion depth 80 should approximate the vertical boron diffusion depth 82. In turn, the lateral diffusion depth 80 should approximate the distance 84 that silicon nitride mask portion 56 "overhangs" the bulk area 72. This relationship is illustrated in Figure 7. After additional cleaning of silicon nitride mask 56, boron is diffused into silicon wafer 52 to the solid solubility limit at the surface 64 of exposed silicon wafer 52. It is desirable to saturate the silicon surface 64 with the maximum possible number of boron atoms. Most preferably a two step boron diffusion will be used wherein the first step saturates the silicon with the desired amount of boron atoms and a second succeeding diffusion step distributes said boron such that a desired distribution of boron atoms in the silicon wafer 52 is achieved. The desired amount of boron atoms will at a minimum be approximately equal the thickness of the deflectable body 12 1x10 19 cm -3 . The final concentration of boron must be at least 1x10 19 cm -3 or the desired structure of deflectable body 12 will not be achieved. Preferably the first diffusion step will take place at approximately 1,000° to 1,200°C for about 0.5 hours to 10 hours in the presence of N 2 and a small amount of O 2 (1 to 10%) and a commercially available source of boron. Most preferably the first step will comprise 6 hours at 1150°C. A preferred boron source is "Boron-Plus" available from Owens-Illinois of Toledo, Ohio, U.S.A. The second boron distribution step is also termed a diffusion step and is preferably a heating process consisting of 0.5 to 10 hours at 900°-1200°C. Most preferably, the second diffusion or distribution step will consist of 3.5 hours at 1000°C.After the removal of silicon nitride mask 56, the surface 58 is anodically bonded to glass substrate. Although those skilled in the art will be familiar with anodic bonding processes, the article, "Mechanisms of Anodic Bonding of Silicon to Pyrex Glass" by K. Albaugh et al., 1988 IEEE Solid-State Sensor and Actuator Workshop Proceeding, 88TH0215-4, pages 109-110, give further details. The bonding of surface 58 to substrate 60 is followed by a bulk silicon etch wherein most of the remaining silicon wafer 52 is removed. This is achieved with the use of an isotropic or anisotropic etchant solution. A suitable etchant will remove the majority of bulk silicon wafer 52. A suitable etchant solution will be a KOH solution comprised of three kilograms of KOH in seven liters of deionized water. The wafer remains in contact with the etchant solution for a period of 1.0 to 5.0 hours at a temperature from 70°-100°C and most preferably 2.5 hours at 90°C for a wafer having an initial thickness of 0.4-0.5 mm. The bulk etch is followed by a selective etch in a solution comprised of three kilograms of KOH in seven liters deionized water saturated with isopropyl alcohol. This solution contacts the remaining wafer until such time as the wafer 52 is clear, i.e. until the glass substrate is visible, followed by an additional 0.5 to 2.0 hours contact with the solution and most preferably 0.75 hours. After the final etching of silicon from the boron doped deflectable upper body 12, conventional photolithographic techniques are used to pattern the desired metal bonding pads and conductors discussed above. Note that prior to the anodic bonding, conventional photolithographic and deposition techniques are used to deposit chromium and platinum on glass substrate to form fixed conductive plates 16 and 18. Although the manufacture of the claimed sense element 8 has been disclosed, it will be appreciated by those skilled in the art that the claimed sense element will subsequently be incorporated into an accelerometer. It is most preferred that the final atmosphere in which the sense element is contained be comprised of at least 95% N 2 gas and be substantially free of water vapor. While the sense element 8 will function in other atmospheres, the N 2 atmosphere ensures that the viscosity of the atmosphere is constant and does not affect the desired response characteristics, particularly the damping ratio. Turning to Figure 1, those skilled in the art will appreciate that more than one of the sense element 8 of the instant invention can be utilized to provide the most preferred, fully differential accelerometer sense element 8. Such a construction allows for the various relative comparisons of the four capacitors such that the use of a reference voltage is unnecessary.

Claims: A capacitive sense element (8) for an accelerometer, comprising: a substrate (10) having a substantially planar upper surface (14) with at least two fixed conductive plates (16, 18) thereon and a substantially planar deflectable body (12) positioned above and parallel to the substrate, the body comprising: a) a pedestal (30) attached to the substrate and surrounded by an internal opening (32) in the body; b) a first upper plate (38) and a second upper plate (40) corresponding to the fixed conductive plates (16, 18) to form a first and a second capacitor; and c) a pair of oppositely directed torsion arms (36) connecting the body (12) to the pedestal (30) and forming a flexure axis around which the first and second upper plates (38, 40) deflect, the total moment of the second upper plate (40) about the flexure axis being greater than the total moment of the first upper plate (38) about the flexure axis such that the body deflects in response to an acceleration normal to the sense element to produce a difference between the capacitances of the first and second capacitors, characterised in that the substrate (10) consists of a dielectric material, and the body (12) consists of boron-doped silicon having a single-crystal structure and has a plurality of openings (50) extending through the upper plates (38, 40), the openings producing a damping effect which modifies the natural frequency and frequency response of the sense element such that the sense element exhibits a damping ratio of from 2 to 20 and a measurement bandwidth of 0 to 5000 Hz. A sense element according to claim 1, wherein the openings (50) extend through the body in a direction perpendicular to the substrate and have individual diameters from 0.1 to 50 µm. A sense element according to claim 1 or claim 2, wherein the first and second upper plates (38, 40) have non-identical masses controlled by the size and number of openings (50) extending therethrough. A sense element according to any preceding claim wherein the damping ratio of the sense element is from 2 to 10. A sense element according to any preceding claim wherein the concentration of boron in the body is greater than 1 x 10 19 cm -3 . A sense element according to any preceding claim wherein the upper edges of the openings are rounded.

Inventors: SPANGLER LELAND JOSEPH, SPANGLER, LELAND JOSEPH
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1994
Main classification: G01P15
IPC codes: H01L29, G01P15, G01P1, G01P21